1. Field of the Invention
The present invention relates to an anamorphic single lens having refracting powers different in the horizontal direction and the vertical direction, which is suitable for laser-applied optical systems in which a light source, e.g. a semiconductor laser or the like, is used.
2. Related Art
Conventionally, in a diffraction limited optical system used for a laser printer, a laser beam is focused to obtain a light spot. Recently, as a light source, a small, light-weight laser diode having an advantage in that the intensity of output light can be directly modulated has frequently been used. However, the structure of the laser diode is such that the radiation angle of the radiated beam is not rotation-symmetrical. Namely, the radiation angle in the vertical direction which is in parallel with the conjunction plane of the semiconductor device and the radiation angle perpendicular to the conjunction plane are different from each other.
FIG. 13 is an illustration which depicts an example of a radiated beam emitted from a laser diode. A light beam emitted from an emission region 3 formed between conjunction planes 2 of a laser 1 has radiation angles .theta..sub.2 and .theta..sub.1 in the vertical direction parallel with the conjunction planes and in the horizontal direction perpendicular to the same, respectively, which have a relationship expressed as .theta..sub.1 =2.theta..sub.2 to 3.theta..sub.2. As a result, a far-field pattern 5 is ellipsoidal. When this radiated beam is made into a parallel light beam by means of a collimator lens, the parallel light beam is inevitably made into an ellipsoidal beam. A spot obtained by focusing such parallel beam by a scan optical system for a laser beam printer is also shaped into an ellipsoidal spot. To solve this problem, a beam shaping optical system using a prism is disclosed
Patent Laid-Open Publication Nos. 60-1750185/1985, 60-191223/1985, where the beam shaping optical system converts an ellipsoidal input beam into a circular beam. The system employs a cylindrical lens or a toric lens to perform beam shaping or to also function as a collimator to obtain a circular parallel beam.
Conventionally, some deflecting systems having a deflective reflecting plane, e.g., scan systems of laser printers using a rotary polygonmirror, are known, in which uneven pitches of scanning lines on a plane to be scanned will not be produced even when a change in light beam deflectively scanned by an inclination of the deflective reflecting plane may occur in a plane perpendicular to a scanning plane. For example, optical systems disclosed in Japanese Patent Laid-Open Publication Nos. 56-36622/1981, 57-144516/1982 are configured with first and second image forming optical systems. The first image forming optical system comprises a collimator lens for collimating a laser radiation beam into a beam having an adequately collimated beam diameter, and a single flat convex cylindrical lens for forming the collimated beam into a linear image. The second image forming optical system comprises a deflecting system having a deflective reflecting plane adjacent to the linear image forming position, and a single spherical lens and a single toric lens for converting the deflected beam into a spot image on a scanned plane. In a plane perpendicular to the scanning direction, the deflective reflecting plane and the scanned plane are in a geometric optical conjugate relation and optically compensate the inclination of the deflective reflecting plane, thereby compensating the uneven pitches of the scan lines.
However, the first image forming optical system, which must be configured with a collimator lens and a cylindrical lens, becomes complicated in configuration and requires a long time for adjustment.
On the other hand, an optical system for an optical disk information recording apparatus is also a diffraction limited system to form a laser beam into a very small spot on a recording plane of an optical recording disk. However, when a laser diode is used as a light source, a rotation-symmetrical radiation beam cannot be obtained as described before. Accordingly, to obtain a collimated beam having a circular cross-section, the conventional system uses a collimator lens having a sufficiently small numerical aperture compared to the beam radiation angle. However, since this method is low in efficiency of light utilization, a large power laser diode must be used, resulting in the increase of cost. To solve this problem, there are known methods of using a coupling optical system which uses a cylindrical lens or uses prisms in combination with a collimator lens. An example is shown in FIG. 14, in which a radiation beam 4 emitted from a laser diode 1 is made into a substantially parallel beam by means of a collimator lens 26, and the diameter of the beam is enlarged merely in one direction by means of prisms 24, 25, whereby a desired circular beam is obtained. This system can improve the light utilization efficiency in comparison to the former method, but is complicated in configuration.
Japanese Patent Laid-Open Publication No. 61-254915/1986 discloses a system which performs these optical functions as described formerly and has a simple configuration. However, this system has too large a quantity of residual spherical aberration or the like to be practically used, because the lens plane used therein is a toric plane of, i.e., an anamorphic lens plane which is configured with arcs expressed only by the curvature radii.